RESIDENTS were still struggling to come to terms with what drove a gunman to go on a rampage through a shopping centre, killing six people.

The killer, Tristan van der Vlis, dressed in camouflage trousers and a bomber jacket, opened fire with an automatic weapon in a parking area on Saturday and walked calmly into Ridderhof mall, in Alphen Aan Den Rijn in the Netherlands, where he continued shooting.

Amongst the six fatalities were a 91-year-old woman and an 80-year-old man -- 17 others were wounded, including two children aged six and 10.

The 24-year-old gun club member then shot and killed himself.

Attack

"This is something you usually see in America, not in the Netherlands," said local resident Martin van der Ploeg.

The shooting spree was the deadliest attack in the Netherlands since a Dutch national drove his car into a crowd in 2009, killing seven people and himself in an apparent attempt to hit the queen in Apeldoorn, 90km east of Amsterdam.

The gunman left a farewell letter, in which he mostly talked about his suicidal feelings. He lived some 200 metres from the shopping centre.

"He was very polite and very nice," said one of Van der Vlis's neighbours, a woman who declined to be identified.

Public prosecutor Kitty Nooy said after questioning members of Van der Vlis' family, investigators had still found no motive for the attack.

"This young man had psychological problems and that was known by a certain number of people," she said.

Ms Nooy said that Van der Vlis had acted alone and was investigated by police in 2003 on suspicion of violating weapons laws but not convicted. He had a licence for five guns and owned three.

As a member of a shooting club, Van der Vlis was allowed to keep a semi-automatic weapon at home, but should not have had access to automatic weapons, investigators said.

Messages of condolence have poured in from members of the public, expressing shock that such an incident could take place in a country with no history of random killings and a relatively low crime rate.

Yesterday, several police stood outside the cordoned-off shopping mall, while people were out on the quiet streets, walking their dogs or cycling.